Determination and distribution of Formation thickness in Baga/Lake sub- basin using seismic imaging and well data, Chad Basin Nigeria *Gabriel Efomeh Omolaiye 1 , Olatunji, Seminu 2 , Jimoh Ajadi 3 , Yusuf Ayoola Jimoh 3 1 Geoslice Solution Limited, Lagos Nigeria. 2 Department of Geophysics, University of Ilorin, PMB 1515, Ilorin, Nigeria 3 Department of Geology and Mineral Science, Kwara State University, Malete, PMB 1530, Ilorin, Nigeria *Corresponding Author: gab_omolaiye@yahoo.co.uk Abstract- The Nigerian part of the Chad Basin also known as Borno Basin to date has been one of the few critically studied basins, with few published scholarly works, compared to other basins such as Niger Delta, Dahomey, etc. The basin is the South-Eastern part of the Mega-Chad basin formed mainly in the Early Cretaceous and part of the Mesozoic-Tertiary rift system in Africa. Deposition of numerous kilometers of clastic sediments of Lower Cretaceous age occurred during the rifting, the sediments are mainly lacustrine shales and sandstones. Active rifting was preceded by thermal subsidence in the late Albian period, permitting the progradation of extensive deltaic and fluvial sediments. This work was undertaken by the integration of 3D seismic interpretations and the well data analysis of eight wells fairly distributed in block A, Baga/Lake sub-basin in Borno basin to determine the thickness of the formations within the sub-basin. Da-1 well (type-well) used in this study was subdivided into stratigraphic units based on the regional stratigraphic subdivision of the Chad basin and was later correlated with other wells using the similarity of observed log responses. The combined density and sonic logs were used to generate synthetic seismograms for seismic to well ties. Five horizons were interpreted, representing the tops of the formations on the 3D seismic data covering the block, average velocity function with maximum error/residual of 0.48% was adopted in the time to depth conversion of all the generated maps. There is a general thickening of sediments from the west to the east and the estimated thicknesses of the various Formations in the Baga/Lake sub-basin are; Chad Formation (400-750 m), Kerri-Kerri Formation (300-1200 m), Fika Formation (300-1000 m) and Gongila Formation (100-1300 m). The thickness of the Bima Formation could not be established because the deepest well (Da-1) terminates within the Formation. This is a modification to the previous and widely referenced studies of over fifty decades. Keyword: Baga/Lake sub-basin, Chad basin. Formation Thickness, Seismic, Velocity Introduction The importance of hydrocarbon to the world economy and the need to increase reserves necessities the Nigeria National Petroleum Corporation to pay more attention to the frontier basins in Nigeria of which the Borno basin is one. The Chad basin is believed to be the botched arm of the triple-junction rift system in the Early Cretaceous leading to the opening of the South Atlantic and the subsequent parting of the South American and African continents (Burke et al., 1970, Ola et al., 2017). The Nigeria Chad Basin also known as Borno Basin is the NE extension of the Benue Trough, with a general NE-SW trend. The basin is made up of three sub-basins namely Gubio and Maiduguri, in the SW, and Baga/Lake, in the NE. The SW sub-basins are also called Upper Benue Trough while the NE sub-basin corresponds to the SW flank of the Termit-Agadem basins located in Niger and Chad (Figure 1a). The 3D seismic technique is an important geophysical tool used to interpret the subsurface geology, the quality of the result is unmatched when integrated with closely or sparsely spaced well data in an area. The tool is very effective for both exploration of frontier basins and the development of brownfields such as mature oil-producing assets. This paper outlines the results of the interpretation of newly acquired 3-D seismic volume constrained with well data with major objectives of (i) interpreting the 3-D seismic cube covering an area of about 3891 Km 2 , (ii) evaluating the well-logs within the seismic cube, (3) performed seismic to well tie, (iv) generating the subsurface maps of the formations with the purpose of accurately estimating the thickness of the formations. Geology of the study area Block A is located in the Baga/Lake sub-basin in the Borno basin. A total of eight wells were drilled in the block, from which five wells are within the valid seismic cube (Figure 1). The block exhibits a typical example of structure and stratigraphy development observed in the Borno basin. The six Formations present in the basin are: the Chad